Fluororesins are excellent, for example, in chemical stability, heat resistance and mold release characteristics as compared with other plastics. Thus, heat shrink tubes made of fluororesins are used as protective and insulating heat shrink tubes in various fields including foods, medicine, pharmaceutics, chemistry, and analytical instruments.
Among uses as general heat shrink tubes for protective and insulating purposes are uses involving processing of products in which the heat shrink tube is once coated, if required, and then only the heat shrink tube is peeled off.
They include, for example, uses in which electric wires, tubes, etc. having a multilayer structure including a protective layer for a blade or the like, or having a heteromorphic structure, are coated with the heat Shrink tube, and the interior members are molded or heat-welded, whereafter the heat shrink tube is removed.
In addition to the excellent properties characteristic of fluororesins, the excellent property of the tube being easily peeled and strippable by hand, namely, peelability, is required of such uses.
Patent Document 1 describes a peelable tube comprising a mixture of a plurality of thermoplastic fluororesins containing different types of fluororesins.
Patent Document 2 and Patent Document 3 show heat shrink tubes having peelability. These documents describe that fluororesins used therein are characterized by fulfilling the change amount of loss energy as specially defined; comprising a mixture of a plurality of fluororesins of different types; and containing a fluororesin accounting for a main proportion in the mixture (i.e., a main fluororesin), the main fluororesin being a polymer composed of at least three types of monomers, the polymer being a copolymer containing at least tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) as its constituent monomers.
The heat shrink tubes of Patent Documents 2 and 3, however, tend to be excellent in peelability, but are decreased in shrinkage rate, as the amounts added of the fluororesins other than the main fluororesin increase. Depending on the uses they are put to, therefore, the amount of their heat shrinkage is insufficient, and they fail to fulfill the roles of heat shrink tubes, so that further improvements are required of them.
The present inventors have also found that the heat shrink tubes described in Patent Documents 1 to 3 show a tendency toward better peelability with increases in the amounts of the other fluororesins added, but pose the problem of decreased transparency such that they become cloudy, making it difficult to confirm the state of the underlying material through the heat-shrunk tube.